This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Field of the Invention
The present disclosure relates to the field of subsurface drilling. More specifically, the present invention relates to an agitator used to release gases entrained in drilling mud returns during a well drilling operation.
Technology in the Field of the Invention
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. Once the wellbore has reached total depth and all casing strings are in place, a string of tubing is typically installed along the casing down to a depth of a designated subsurface formation. The tubing string may either be production tubing or injection tubing. A well head is fixed at the surface above the wellbore for suspending the tubing, controlling wellbore pressures, and directing the flow of fluids into or out of the wellbore, all as part of hydrocarbon recovery operations.
During the drilling process, a drilling fluid is continuously pumped into the wellbore. Drilling fluids are typically made up of clays and chemical additives which are carried in an oil or water base. The drilling fluid, sometimes referred to as “mud,” is pumped down the drill string and to the drill bit at the lower end of the wellbore. The fluid is further pumped through openings or nozzles in the drill bit where the fluid picks up rock chips. The fluid and carried chips are then pumped back to the surface via an annulus residing between the drill pipe and the surrounding subsurface rock matrices.
The primary function of the mud is to cool and lubricate the drill bit as rock is being cut. However, the mud also carries drill cuttings up and out of the well and to the surface. Samples of the drill cuttings may be collected at shale shakers and analyzed during a drilling process. Also, the mud maintains a hydrostatic pressure within the wellbore which prevents pressurized fluids in subsurface formations from blowing out through the borehole. This situation is known as a “kick.”
During the drilling operation, the drill bit will penetrate one or more zones of comparatively high pressure adjacent the formations being drilled. In some subsurface formations, hydrocarbon gases under pressure will invade the wellbore. Those gases will typically include at least methane and ethane, and will frequently also contain carbon dioxide and/or hydrogen sulfide and/or nitrogen. The gas constituents become entrained in the drilling fluid as the drill bit penetrates the formation.
As the drilling fluid returns to the surface, it carries information about the nature of the formations being drilled. This information resides in the gases and rock chips held in the mud. Service companies are frequently retained to analyze drill cuttings and capture gases that break out of the drilling mud solution once the mud has returned to the surface. By examining the cuttings for traces of hydrocarbons, and by examining the quantity and type of gases released, a petroleum geologist or lab chemist may determine the likelihood of producing oil and/or gas from the well, and at what depths.
In the case of gas, gas is typically extracted from the mud by mechanical agitation using a so-called gas trap. The gas trap may be located in a possum belly, or “header tank,” at the rig. A possum belly is a metal container at the head of the shale shaker. The possum belly is connected to the return flow line at the surface and slows the flow of fluids after they have gained momentum from coming down the flow line. This, in turn, prevents the drilling fluids from shooting off of the shale shakers.
In some instances, the gas trap is in a box adjacent the shale shakers. More frequently, the gas trap is in the return mud tank or pit beyond the shale shakers. Some of these gas traps include beaters or agitators that cause gas to break out of solution.
In any arrangement, the captured gas is analyzed for hydrocarbons and/or total gas content using one or more detectors. Known detectors include catalytic combustion detectors (CCD), thermal conductivity detectors (TCD) and flame ionization detectors (FID). Separation and quantification of the different hydrocarbon gases (e.g. methane through pentanes) are then typically carried out via gas chromatography techniques with similar or different detectors.
U.S. Pat. No. 7,741,605 issued in 2010 discusses techniques for analyzing gases that are released from a gas trap. This patent, entitled “Method and Apparatus for Detecting Gas Conveyed by Drilling Fluids,” was assigned to Varco I/P, Inc. The '605 patent is incorporated by reference herein in its entirety.
For so-called gas trap agitators, a challenge exists with respect to the placement of the gas trap. In this respect, the gas trap needs to be substantially submerged in the fluid return tank (such as a mud tank) so that the agitator is able to stir the return fluids, thereby encouraging gas breakout. At the same time, if the return fluids get too high in the tank, the fluids can interfere with the release of gases into the riser above the agitator. Positioning the gas trap becomes more difficult due to constant changes in fluid viscosity and density in the returns.
Therefore, a need exists for an improved gas trap that is able to accommodate changes in fluid levels in a mud tank. Further, a need exists for a method of capturing gas returns using a gas trap that is able to float in the tank, thereby adjusting for variations in depth, weight and viscosity of the drilling mud during the drilling process.